Hitherto, WC base sintered alloy has been widely used as a tool for cutting steel or cast iron and as an abrasion and impact resisting tool, because of its excellent properties such as abrasive resistance, heat resistance, corrosion resistance, etc. Because of the recent shortage in the natural source of tungsten, however, new materials such as cermets, ceramics and sintered diamond have been developed for use in place of WC tools and put to some practical use.
Among these new materials, Al.sub.2 O.sub.3 base ceramics such as the so-called white ceramics and black ceramics have been widely used as tools for cutting cast iron at high speed.
But, Al.sub.2 O.sub.3 base ceramics do not present high mechanical properties, particularly they do not present sufficiently excellent properties with respect to fracture toughness and heat impact resistance (because of the low heat conductivity, Al.sub.2 O.sub.3 ceramics involve cracks due to thermal stress). Thus, it is difficult to prepare a cutting tool of long life time and high reliability. Accordingly, Al.sub.2 O.sub.3 base ceramics have been employed only in a restricted usage.
On the other hand, non-oxide ceramics such as nitride, carbide, silicate, etc have been developed and become the object of the attention of the cutting tool manufacturers. Among them, silicon nitride (which will be referred to as "Si.sub.3 N.sub.4 " hereinafter) base ceramics present prominently excellent properties regarding fracture toughness and heat impact resistance, etc., and the chipping amount thereof during cutting operation is ordinarily low. Thus a cutting tool of Si.sub.3 N.sub.4 having a life time sufficiently long for practical use is now being developed.
Si.sub.3 N.sub.4 base ceramics present, however, the following problems:
First, in the case of steel cutting, the cutting tool of Si.sub.3 N.sub.4 suffers from abnormal chipping and abrasion due to the reaction between Si and Fe.
Second, compared with Al.sub.2 O.sub.3 base ceramics, Si.sub.3 N.sub.4 base ceramics present a low abrasive resistance under high speed cutting conditions.
For overcoming these defects, there has been proposed, in Japanese Patent Laid-Open No. 1308/1979, a process for applying Si.sub.3 N.sub.4 with a coating of ceramic having a high abrasive resistance such as Al.sub.2 O.sub.3 etc.
Although the above prior art process is effective to some extent, the adhesion between the substrate and the coating is not sufficiently strong and thus there remain some problems to be resolved.
In the prior art processes, Si.sub.3 N.sub.4 is hot pressed together with a sintering agent such as Al.sub.2 O.sub.3, MgO and Y.sub.2 O.sub.3. Otherwise Si.sub.3 N.sub.4 is mixed with Al.sub.2 O.sub.3, AlN, Y.sub.2 O.sub.3, etc., so that the so-called sialon compound (which is believed to be composed mainly of Si-Al-O-N) is formed. The mixture is then sintered or hot pressed in a nitrogen atmosphere. These prior art processes, however, necessitate the addition of Al.sub.2 O.sub.3 and is not sufficiently effective to provide a cutting tool of long life time.